Bleaching is a common method for increasing the whiteness of pulp. Industry practice for improving appearance of fluff pulp is to bleach the pulp to ever-higher levels of brightness (the Technical Association of the Pulp & Paper Industry (“TAPPI”) or the International Organization for Standardization (“ISO”)). However, bleaching is expensive, environmentally harsh and often is a source of manufacturing bottleneck. Widespread consumer preference for a brighter, whiter pulp drives manufacturers to pursue ever more aggressive bleaching strategies. While highly bleached pulps are “whiter” than their less-bleached cousins, they are still yellow-white in color. A yellow-white product is undesirable. Countless studies suggest that consumers clearly favor a blue-white over a yellow-white color. The former is perceived to be whiter, i.e., “fresh”, “new” and “clean”, while the latter is judged to be “old”, “faded”, and “dirty”.
While bleaching directly elevates brightness, it only indirectly elevates whiteness. Due to the latter, bleaching is not always the most efficient method for boosting product whiteness. For example, even after aggressive bleaching, a product's whiteness can always be extended beyond that achievable with bleaching alone by judicious addition of colorant.
The addition of small amounts of blue colorant to improve whiteness appearance is known in other fields, such as papermaking. However, the practice is unknown to the fluff pulp community. The addition of colorant to fluff pulp to improve its whiteness properties is novel to the industry.
The practice of pre-coloring papermaking pulp is not usually done nor is it necessarily desired. With the former, intentional alteration of optical properties often ends up degrading product specifications such as TAPPI brightness, which is undesirable. With the latter, one runs the risk that colorants may not survive the unpredictable manufacturing environments of downstream processes. This is because previously applied colorant can be adversely affected chemically and/or physically during post-processing operations resulting in unexpected or undesirable color changes or even full loss of color. Furthermore, some colorants can be lost or rendered ineffective during various post-processing operations disrupting process health and reliability. Therefore, any optical enhancement is usually accomplished by addition of tinting colorants, fillers, and/or fluorescent dye during the papermaking stage. A process for enhancing the whiteness, brightness, and chromaticity of papermaking fibers has been described in U.S. Pat. No. 5,482,514. The process relates to adding photoactivators, particularly water-soluble phthalocyanines, to papermaking fibers to enhance their optical properties by a catalytic photosensitizer bleaching process. The resulting bleached papermaking fibers can be advantageously incorporated into paper sheets.
With fluff pulp, as well as most pulp and paper products, TAPPI brightness serves as the de facto standard in lieu of an industry-specific whiteness specification such as CIE Whiteness (Commission Internationale d'Eclairage). Because of this, brightness serves two key roles. First, brightness is a manufacturing parameter. Second, brightness is a specification for classifying finished product grades. The implicit, but dubious, assumption to this day has been that brightness is equivalent to whiteness. Common papermaking practice is to either add blue tinting dyes or tinting pigments and/or different types of blue-violet fluorescent dyes to boost whiteness properties. Tinting colorants are either finely ground colored pigments suspended in a dispersant or synthetically produced direct dyes. Tinting dyes have some affinity to cellulose while tinting pigments have little to none. Fluorescent whitening agents (FWA) used in the pulp and paper industry are of three types: di-, tetra-, or hexasulphonated stilbene compounds. These chemicals require ultraviolet (UV) light to excite fluorescence. While there is strong UV content in daylight, even common office lights produce enough UV light to permit some excitation.
Accordingly, there exists a need for pulp having improved whiteness. A need also exists for a method for making whitened pulp. The present invention seeks to fulfill these needs and provides further related advantages.